Lignin is the major noncarbohydrate constituent of wood and functions as a natural plastic binder for the cellulose fibers. Lignin can be removed from wood by either the sulfite cooking process or the alkaline cooking process.
It is known that the rate of delignification is proportional to the amount of lignin present in the wood, the chemical pulping reagent concentration present in the wood during the delignification process, and the temperature dependent reaction rate, k. It is further known that the rate of delignification for pulping varies with the temperature in accordance with the Arrhenius equation. From this equation, the temperature dependent reaction rate, k, can be determined and subsequently utilized to determine the "H factor" and the Kappa Number for the delignification process being utilized.
The prior art is primarily concerned with processes associated with wood pulping delignification. For example, U.S. Pat. No. 3,523,060 (Herdel, et al) discloses a modification of the sulfite pulping process wherein a very large quantity of sulfur dioxide is utilized and the delignification process is forced by using a very high temperature.
The Leithem patent (U.S. Pat. No. 4,295,929) is directed to the same sulfite delignification process, however, in this reference the proportion of combined sulfur dioxide used in the digestion process is varied as a function of the rate of heating. In essence, this reference teaches that an increase in the proportion of sulfur dioxide used in the digestion process combined with an increase in the heating rate permits a shortening of the total digestion time. Thus, the Herdel, et al and the Leithem references are directed to variations of the sulfite digestion process in order to shorten the total digestion time.
The Somer, et al patent (U.S. Pat. No. 2,545,389) discloses apparatus for increasing the sulfur dioxide content of the cooking acid used in the process. There is an inverse relationship between sulfur dioxide content and total digestion time, and thus, this reference is directed to the apparatus for increasing the sulfur dioxide content of the cooking acid rather than to the process itself.
It is known that the foregoing principles of sulfite cooking also apply to alkaline cooking. Regardless of the type of cooking utilized, the rate of delignification can be determined and the temperature dependent reaction rate, k, can be integrated over time to produce a single parameter, the H factor, to describe the combination of cooking times and temperatures in conjunction with the kinetic principles of pulping. The H factor is related to the Kappa Number, K, which is a measurement of the degree of cooking.
The implementation of the known background art is done as follows. Typically, pulp is manually sampled from the process periodically and analyzed for the degree of delignification per a standardized lab test procedure. The test result index, pulp Kappa Number, is reported to operations as a guide for manual adjustment of active chemical addition or the time/temperature profile.
Also important is the residual chemical concentration of the spent liquor from the cooking process. This residual chemical has significant impact on total mill operation and economics. Although this process variable may be measured via a conductivity sensing device or sampled from the process for lab analysis, it is normally not included in the manual feedback mechanism. Further, it is difficult for operations, given the large array of variables, to assess the quality parameters for an appropriate adjustment and solve the process interactions manually. Prior art does not incorporate residual chemical as a controlled variable into a control policy for the delignification process.
In view of the foregoing, it has become desirable to develop a method of modeling for controlling the delignification process utilizing the foregoing parameters.